1. Field of the Invention
The present invention relates to a method and an apparatus for the determination of the average particle size in a slurry by using ultrasonic radiation.
2. Description of the Prior Art
Ultrasonic sound is attenuated in a slurry by losses due to viscosity and scattering. The attenuation is dependent primarily on the size of the slurry particles, their proportion of the volume, and the frequency used. The specific gravity of the particles also has some effect. In addition to attenuation the share of scattering can be measured directly by placing the receiver outside the radiation cone of the transmitter-sensor. By combining the data concerning scattering and attenuation, the effect of the density of the slurry can in theory be totally eliminated and an average particle size can be obtained which weights the various size categories approximately by their proportions of the volume. If the shape of the particle size distribution is approximately retained, the result can further be converted to the sieve residue percentage.
Attenuation of ultrasonic sound and the related scattering have been studied quite extensively even in connection with particle size measuring. Applications of direct measuring of scattering are rarely described, but the theory has been treated rather widely in the literature.
U.S. Pat. No. 3,779,070 to Cushmon et al. uses two ultrasonic sound frequencies, which are selected according to the criteria given in the specification and in such a manner that both the particle size and the solids content of the slurry can be determined. Even though, in some cases, one of the frequencies is selected so that the predominant mechanism determining the attenuation is scattering, this known method is characterized in that the only phenomenon that is detected and treated further in the method is the radiation intensity which has passed through and not the scattered ultrasonic radiation.